dm: timer: Implement pre_probe()

Every timer device needs to have a valid clock frequency and it
can be specified in the device tree. Use pre_probe() to get this
in the timer uclass driver.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Thomas Chou <thomas@wytron.com.tw>
Acked-by: Simon Glass <sjg@chromium.org>